This invention relates to a method of controlling the stirring strength of a jet of gas such as oxygen gas as well as the flow rate of the gas independently of each other without changing the height of a lance in a top-blowing oxygen furnace for producing steel, a top-blowing oxygen furnace for making copper, a top-blowing oxygen furnace for gasifying coal, and the like, each having a molten metal bath therein (hereinafter collectively referred to as "top-blowing oxygen furnace"). In particular, this invention relates to a method of increasing or decreasing the stirring strength of a jet of oxygen gas while maintaining at a given level the flow rate of the oxygen gas blown onto the surface of a molten metal bath without changing the height of an oxygen top-blowing lance. Alternatively, this invention relates to a method of increasing or decreasing the flow rate of a jet of oxygen gas without changing the stirring strength of the oxygen gas.
It is well known that the stirring strength of a jet of oxygen gas blown through a top-blowing lance has a great influence on the stirring of a molten metal bath and on gas-liquid interface reactions in the molten metal bath. For example, under the same conditions, the reactions at the interface proceed rapidly when the stirring strength of a jet of oxygen gas increases. On the other hand, the interface reactions becomes moderate when the stirring strength of a jet of oxygen gas decreases. Furthermore, oxidation reactions proceed rapidly when an oxygen gas supply increases, and even a reducing reaction sometimes occurs when the flow rate of oxygen gas decreases.
Therefore, the main controlling factors of the operation of a top-blowing oxygen furnace are the stirring strength of a jet of oxygen gas and the supply of oxygen gas. Thus, numerous measures have been proposed to control the stirring strength and the supply of a jet of oxygen gas. The supply of oxygen gas can be controlled very easily merely by controlling valves. However, when the flow rate of oxygen gas is changed, the stirring strength of a jet of the oxygen gas is also changed and vice versa, making it difficult to control the supply of oxygen gas independently of the stirring strength of a jet of the oxygen gas.
The "stirring strength of a jet of oxygen gas" means the strength of a jet of oxygen gas blown onto a molten metal surface to stir a molten metal bath and can be described as follows, presuming that the position of a lance is fixed.
Stirring Strength to a Molten Metal Bath EQU .varies.(Oxygen Gas Jet Momentum).times.(Lance Height from the Surface of Molten Metal Bath)-3 (I)
Oxygen Gas Jet Momentum EQU .varies.(Oxygen Gas Flow Rate).times.(Oxygen Gas Jet Speed at the Outlet of Lance) EQU .varies.(Pressure of Oxygen Gas at the Inlet of Lance) (II)
From the above Formulas (I) and (II),
Stirring Strength to a Molten Metal Bath EQU .varies.(Pressure of Oxygen Gas at the Inlet of Lance) EQU .times.(Lance Height from the Surface of Molten Metal Bath)-3(III)
As is apparent from the above Formula (III), the following methods have been employed in the prior art so as to control the stirring strength of a jet of oxygen gas: (a) a method of sliding up and down a lance through which oxygen gas is supplied; (b) a method of changing the momentum of a jet of oxygen gas, i.e., changing the pressure of the oxygen gas supplied, which inevitably results in a change in the flow rate of the oxygen gas.
However, Method (a) above has disadvantages that a lance has to be replaced frequently since the tip of the lance is vigorously attacked by splashed molten metal and molten slag when the lance is lowered down toward the molten metal bath surface. In addition, a lance-sealing mechanism which assists the movement of the lance has to resist the bending which occurs during operation, making the sealing apparatus very expensive. Maintenance costs are also expensive.
On the other hand, in Method (b) above, since a change in pressure results in a change in flow rate, a change in reaction conditions is inevitable, and it is impossible to control the stirring strength of a jet of oxygen gas effectively without changing the flow rate of the oxygen gas.
It has been thought that it is impossible to control the flow rate and the stirring strength of oxygen gas independently of one another. Namely, it is impossible to increase or decrease the supply of oxygen gas without changing the stirring strength. Nor is it possible to increase or to decrease the stirring strength of a jet of oxygen gas without changing the flow rate of the oxygen gas supplied through a lance. Therefore, it is necessary to vary the height of the lance in order to control the operation of a top-blowing oxygen furnace.
For example, in refining a low carbon steel with a top-blowing oxygen converter, it is necessary to reduce the flow rate of oxygen gas in order to prevent an excess oxidation of molten iron at a finishing stage of refining. However, if the flow rate of oxygen gas is reduced, the stirring strength is also reduced. Therefore, a lance has to be lowered toward the surface of a molten metal bath in order to maintain the stirring strength.
In addition, in the case of a gasification furnace, sometimes it is necessary to suppress the stirring of a molten metal bath to some extent in order to reduce the thermal damage to the furnace body. In such a case it is necessary to either decrease the flow rate of oxygen gas or to raise the lance. However, if the flow rate of oxygen gas is decreased, the capacity of a gasification furnace is decreased. Therefore, the lance has to be raised, since the capacity of the gasification furnace cannot be reduced in order to maintain a given output of gas.